Cerium-based abrasive material and method for preparation thereof

ABSTRACT

A cerium-based abrasive and a production method of the cerium-based abrasive excellent in polishing properties of a high polishing speed and scarce formation of polishing scratches are provided by keeping the color of the abrasive in specified ranges or stably making fluorine be contained in the abrasive. For example, as such a cerium-based abrasive, examples include a cerium-based abrasive containing cerium oxide as a main component and having an L* value in a range not lower than 65 and or lower 90, an a* value in a range 0 or higher but 15 or lower, and a b* value in a range 10 or higher but 30 or lower in the case the color is expressed by an L*a*b* color system.

TECHNICAL FIELD

[0001] The present invention relates to a cerium-based abrasive and aproduction method of the cerium-based abrasive, more particularly, acerium-based abrasive and a production method of the cerium-basedabrasive excellent in polishing properties such as polishing speed-andpolished surface.

BACKGROUND ART

[0002] Recently, an abrasive is employed for a variety of purposes.Above all, highly precise surface polishing is required for glasssubstrates such as a glass substrate for an optical disk, a glasssubstrate for a magnetic disk, a glass substrate for a photomask to beemployed for semiconductor fabrication, a glass substrate for LCD andthe like, an optical lens and the like.

[0003] For surface polishing for them, rare earth oxides, especially, acerium-based abrasive containing cerium oxide as a mainly component(hereinafter referred as to a cerium-based abrasive) has been employed.Because as compared with zirconium oxide and silicon dioxide, ceriumoxide is excellent in polishing properties for polishing glass. Forexample, since it has a high polishing speed as compared with zirconiumoxide, silicon dioxide, aluminium oxide, and the like, cerium oxide iscapable of performing polishing faster. Further, since cerium oxide hasa hardness not so high, smoother glass surface can be obtained afterpolishing by using cerium oxide for polishing.

[0004] However, the polishing properties of the cerium-based abrasiveare determined depending on a large number of factors such as contentsof cerium oxide, fluorine (F) and the like, a specific surface area andthe like. Consequently, to control the polishing properties, there areproblems that the respective properties have to be precisely evaluatedand comprehensively judged.

[0005] Further, the production conditions of the cerium-based abrasiveare also important. For example, as one of most important steps, thereis a roasting step in the latter half of the production process. Theroasting step is a step of heating raw materials for the cerium-basedabrasive at a high temperature in an oxidative atmosphere and in thestep, fluorine, one of important factors determining the polishingproperties, is easy to be dissipated from raw materials. Consequently,the roasting step has to be properly controlled. Especially, in the caseof using an apparatus capable of carrying out continuous roasting, it isdifficult to quickly and precisely judge the roasting state, therebyleaving difficulty in control as a problem.

[0006] The invention is developed to deal with such problems and aims toprovide a cerium-based abrasive and a production method of thecerium-based abrasive excellent in polishing properties.

DISCLOSURE OF THE INVENTION

[0007] In polishing using a cerium-based abrasive, the following arerequired: the polishing speed is high (the polishing efficiency is high)and at the same time no scratch is formed and further the remainingabrasive after polishing is excellent to be cleaned out. In order toachieve such purposes, various physical values such as the size of theabrasive particles and chemical compositions have been investigated andvariously tried to be optimized. No adhering to the viewpoints of theproperties of an abrasive which have been thought ever before, inventorsof the invention have made various investigations regarding abrasivesevaluated to be excellent in polishing evaluation. As a result, it hasbeen found the color of a cerium-based abrasive and the polishingevaluation have a correlation. Further investigations have consequentlymade it clear that an excellent abrasive can be obtained by optimizingthe color of the cerium-based abrasive to complete the invention.

[0008] In a cerium-based abrasive containing cerium oxide as a maincomponent, the invention has a characteristic that the cerium-basedabrasive has an L* value 65 or higher and 90 or lower when a color ofthe cerium-based abrasive is expressed in an L*a*b* color system.

[0009] The color system L*a*b* (L-star, a-star, b-star) defined in JIS Z8729 is very frequently used in the industrial field for colormanagement and a color is expressed by a value of L*, a value of a*, anda value of b* (hereinafter referred as to L* value, a* value, and b*value, respectively). L* expresses brightness and is also called as“brightness index”. On the other hand, a* and b* express hue andchromaticness and are also called as “chromaticness index”. In theL*a*b* color system, as an L* value increases, a color becomes closer towhite and as decreases, closer to black. Also, as an a* value isincreased more in the plus side, a red type color is intensified moreand as decreased more (increased more in the minus side), a green typecolor is intensified. Also, as a b* value is increased more in the plusside, a yellow type color is intensified more and as decreased more(increased more in the minus side), a blue type color is intensified.Also, if both a* value and b* value are zero together, it meanscolorless.

[0010] As a result of investigations, it has been found that, for acerium-based abrasive, the state with a low L* value is a stateparticles of the abrasive are excessively grown and also a state withmany coarse particles causing scratch formation at the time ofpolishing. If an abrasive in such a state is used, polishing scratchesare easy to be formed, resulting in impossibility of obtaining asmoothly polished surface. On the other hand, it has also been foundthat the state with a high L* value is a state the particle growth byroasting is insufficiently promoted, resulting in impossibility ofobtaining a sufficiently high polishing speed, difficulty of evendissipation in a dispersant, and easiness of scratch formation owing toremaining huge agglomerated particles. As a result of suchinvestigations, it is found that a cerium-based abrasive with an L*value of 65 or higher and 90 or lower, preferably 70 or higher and 80 orlower, hardly causes scratches and is excellent in polishing propertiessuch as smoothness.

[0011] The a* value in the L*a*b* color system is preferably 0 or higherand 15 or lower.

[0012] That is because if the a* value is lower than the range, thecontent of fluorine is so low as to make it impossible to cause chemicalreactions needed at the time of polishing glass and make the extremelysmall rough state of the surface to be polished smooth. On the otherhand, that is also because if the a* value is higher than the range, yetthe polishing effect is high, the abrasive excessively contains fluorineand it is not preferable. Further, the content of the coarse particlesis high to result in easiness of polishing scratch formation. That issupposedly attributed to the particle growth was excess at the time ofroasting. From such viewpoints, the a* value is preferably 5 or higherand 15 or lower.

[0013] The b* value in the L*a*b* color system is preferably 10 orhigher and 30 or lower.

[0014] That is because if the b* value is lower than the range, thecontents of fluorine and praseodymium oxide are so low as to make itimpossible to cause chemical reactions needed at the time of polishingglass and make the extremely small rough state of the surface to bepolished smooth. On the other hand, that is also because if the b* valueis higher than the range, yet the polishing effect (the polishingefficiency) is high, the abrasive excessively contains fluorine and isin a state that many coarse particles to be a cause of the polishingscratches are formed and it is not preferable. That is supposedlyattributed to the particle growth was excess at the time of roasting.From such viewpoints, the b* value is preferably 20 or higher but 25 orlower.

[0015] As it is made understandable from above descriptions, acerium-based abrasive with an L* value of the color system L*a*b* 65 orhigher and 90 or lower, an a* value 0 or higher and 15 or lower, and ab* value 10 or higher but 30 or lower has a sufficiently high polishingspeed, scarcely causes polishing scratches, scarcely leaves a remainingabrasive, and is stably provided with these excellent polishingproperties. Consequently, such an abrasive is suitable to be used forprecision polishing for which surface smoothness after polishing isespecially required. It is further preferable if the L* value is 70 orhigher but 80 or lower, the a* value is 5 or higher but 15 or lower, andthe b* value is 20 or higher but 25 or lower. Incidentally, the L*value, the a* value, and the b* value can be recognized with eyeobservation by a person or a measurement apparatus. Consequently, forexample, the quality can be judged or controlled by specifying the colorof an abrasive by comparing the color of the cerium-based abrasive withthat of a standardized product or the quality can be judged orcontrolled by specifying the color by employing a measurement apparatus.

[0016] Incidentally, the color control to make a cerium-based abrasivehave a desired color can be actualized by controlling the content offluorine and further controlling the content of praseodymium oxide. Forexample, the color of an abrasive powder is measured in the roastingstep in the production process and the roasting temperature, theroasting duration, the gas circulation state in a roasting furnace, andthe supply speed to the roasting furnace are adjusted to easily keep theL* value, the a* value, and the b* value in specified ranges,respectively, in the L*a*b* color system. The polishing propertiesregarding the polishing speed, the polishing scratches, the remainingabrasive and the like can easily be controlled by keeping the color ofthe cerium-based abrasive in the above-described specific ranges.

[0017] The cerium-based abrasive contains, as rare earth oxides otherthan cerium oxide (CeO₂), lanthanum oxide (La₂O₃), neodymium oxide(Nd₂O₃), praseodymium oxide (Pr₆O₁₁), samarium oxide (Sm₂O₃), and thelike. Since rare earth elements are similar in chemical and physicalproperties to one another, they are difficult to be separated. However,they do not significantly inhibit the effect as an abrasive. Those to beused as abrasives are those containing cerium oxide as a main componentand generally, the ratio, that is, the content, of cerium oxide in thetotal rare earth oxides (hereinafter, abbreviated as TREO) is requiredto be 30% by weight or higher. Because if the content of cerium oxide inTREO is less than 30% by weight, a sufficient polishing speed cannot beobtained to result in insufficiency for practical use. If the value is30% by weight or higher, practical use for polishing, especially forpolishing glass is possible.

[0018] Further, in the cerium-based abrasive, other than rare earthoxides, there remain oxides of such as Si, Al, Na, K, Ca, Ba and thelike or compounds other than oxides contained in minerals such asbastnasite. Some of them have a function as an abrasive, yet thefunction is low, and become a cause of polishing scratches. For that,the content of TREO in the cerium-based abrasive is controlled to be 80%by weight or higher but 99% by weight or lower. If TREO is less than 80%by weight, many polishing scratches are formed. If TREO exceeds 99% byweight, the productivity is lowered in order to increase the purity.

[0019] Further, it is important to contain a fluorine component as thecerium-based abrasive, especially as an abrasive for glass. That is,because it is supposed to be probable that existence of the fluorinecomponent makes chemical polishing possible to increase the smoothnessof the polishing surface.

[0020] As a result of such investigations, the cerium-based abrasive ofthe invention is found preferable to have the content of TREO in itself80% by weight or higher but 99% by weight or lower and the content offluorine in TREO 0.5% by weight or higher but 10% by weight or lower.

[0021] Because it is found if the content of fluorine in TREO is 0.5% byweight or higher, an excellent polishing effect especially for use forpolishing glass can be obtained owing to not only simply physicalpolishing but also chemical reactions by fluorine. Because, in otherwords, if the content of fluorine is less than 0.5% by weight, thepolishing speed is too low for practical use. Further if the content offluorine exceeds 10% by weight, an interior of a roasting furnace isvulnerable to corrosion and the cost for waste-gas treatment will behigher, disadvantageously.

[0022] Incidentally, the content of fluorine is a value calculated byquantitatively measuring fluorine of the same specimen as a specimen ofwhich TREO is measured and calculating the weight as the content inTREO. The reason for doing that is because almost all of fluorine islost at the time of TREO measurement and therefore even if fluorine ismeasured in a specimen obtained after the TREO measurement, no precisevalue can be obtained.

[0023] The cerium-based abrasive is preferable to have a content ofpraseodymium oxide contained in TREO 1% by weight or higher but 10% byweight or lower.

[0024] As described above, it is important for the cerium-basedabrasive, especially as an abrasive for glass, to contain a fluorinecomponent. Consequently, if dissipation of fluorine at the time ofroasting proceeds to the extent too far, the probability that thefluorine component contained in the abrasive is too slight is high andit is not preferable. An investigation of the roasting step has,therefore, been made to find difference exists in the degree of thedissipation of the fluorine component even if the roasting conditionsare made constant in the roasting step.

[0025] Hence, the chemical behavior of rare earth elements other thancerium, and fluorine at the time of roasting has been investigated. As aresult, a variety of light rare earth elements other than cerium arefound having the effectiveness of the effect to prevent the dissipationof fluorine in the roasting step and among them, especially, existenceof praseodymium or praseodymium oxide is excellent in thedissipation-preventive effect on the fluorine. For example, in the casefluorine is immobilized in form of a fluorine-containing compound ofpraseodymium, a chemical polishing function in polishing, supposedlyderived from the fluorine component, can be obtained and consequently, ahigh polishing speed can be obtained and excellent surface smoothness isobtained.

[0026] The reasons for the excellent dissipation-preventive effect aresupposedly as follows. At first, praseodymium in a form of praseodymiumcompounds (e.g., praseodymium oxide), the oxidation number can be notonly +3, which is common in the case of rare earth elements, but also avalue higher than +3, and especially a value higher than +3 is stable inthe case of praseodymium oxide: praseodymium forms a compositecomposition of the oxide and the fluoride and it is thermally stable:and the like.

[0027] From such investigation results, praseodymium compounds includingpraseodymium oxide were found preventing the dissipation of fluorine atthe time of roasting. And the particle growth of raw materials for thecerium-based abrasive is found possible to be controlled by adjustingthe content of praseodymium oxide to be a proper ratio. Hence, thecontent of fluorine and the content of praseodymium oxide necessary forpolishing have further been investigated.

[0028] As a result, the content of praseodymium oxide is found to bepreferably, as described above, 1% by weight or higher and 10% by weightor lower. Because if the content of praseodymium oxide is less than 1%by weight, the dissipation of fluorine at the time of roasting cannot beprevented. Further, in the case the content is low, the particle growthsometimes does not proceed and the obtained cerium-based abrasive doesnot have a sufficiently high polishing speed. Further, in order toadjust the content of praseodymium oxide to be lower than 1% by weightin the abrasive prediction steps, a special treatment is required toremove praseodymium compounds including praseodymium oxide to cause aproblem in terms of the cost. On the other hand, if the content ofpraseodymium oxide exceeds 10% by weight, immobilization of fluorine inthe roasting step is sufficient to obtain a sufficiently high polishingspeed, however there occurs a problem that the particle growth proceedsexcessively. Further, since the color of the cerium-based abrasivebecomes too dark owing to the excessively high content of praseodymiumoxide, the difference becomes difficult to be detected by colorimetricresults, especially based on the a* value to make quality control ofabrasives by colorimetry difficult. Further, as compared with ceriumoxide, praseodymium oxide has high hydrophilicity and in the case of anabrasive slurry, there occurs a problem that the polishing speed ischanged owing to the alteration of pH of the slurry.

[0029] Further, the cerium-based abrasive is preferable to have thespecific surface area of abrasive particles 1 m²/g or higher but 30 m²/gor lower.

[0030] If the specific surface area is lower than 1 m²/g, yet thepolishing speed is sufficiently high, there exists a problem that thepolishing precision is low and polishing scratches are many. If thespecific surface area exceeds 30 m²/g, if a slurry in which an abrasiveis sufficiently dispersed is used, a smoothly polished surface can beobtained after polishing, however preparation of a sufficientlydispersed slurry itself is difficult. Further, since a sufficientpolishing speed cannot be obtained, the work efficiency of polishingglass or the like is low to make practical use difficult. Further, sincethe amount of the remaining abrasive increases, it is a problem to useas an abrasive for precision polishing.

[0031] From such a point of view, if the value of the specific surfacearea is kept in the above-described range, an abrasive obtained isexcellent in polishing properties; a sufficiently high polishing speed,less formation of polishing scratches at the time of polishing,capability of giving smoother polished face after polishing with littleremaining of the abrasive on the polished face after polishing.

[0032] Incidentally, the cerium-based abrasive according to theinvention may be used as an abrasive slurry by being mixed with anddispersed in water or an organic solvent. The concentration of thecerium-based abrasive (the solid matter) in the abrasive slurry ispreferably 1% by weight or higher but 40% by weight or lower, furtherpreferably 5% by weight or higher and 30% by weight or lower. Examplesas the organic solvent usable are alcohol, a polyhydric alcohol,tetrahydrofuran and the like. The reason for the setting of thecerium-based abrasive to be 1% by weight or higher but 40% by weight orlower is because if it is lower than 1% by weight, it causes undesirableresults that the polishing efficiency is low owing to the lowconcentration and a large quantity of polishing waste solution isgenerated as well. On the other hand, it is also because if it exceeds40% by weight, the viscosity of the slurry becomes high to makequantitative and constant supply of the slurry difficult to result inuneven polishing and it is also not preferable.

[0033] The abrasive slurry may further contain additives such as adispersing agent, a solidification-preventive agent, a pH adjustingagent and the like. As the additives, examples are sodiumhexametaphosphate, sodium pyrophosphate, crystalline cellulose, calciumsecondary phosphate, sodium β-naphthalene sulfonate-formalin condensate,synthetic silicon dioxide, polyacrylic acid salt such as polyacrylicacid sodium salt, carboxymethyl cellulose, polyethylene oxide, polyvinylalcohol and the like. These additives may be mixed with the cerium-basedabrasive at first and made to be slurry of the cerium-based abrasivecontaining the additives; or the additives may be dissolved or dispersedat first in water or an organic solvent and then the cerium-basedabrasive is dispersed; or the additives may be added when thecerium-based abrasive is made to be a slurry using water or an organicsolvent. The weight of the additives is generally 0.1% by weight orhigher but 4% by weight or lower to the weight of the cerium-basedabrasive (the solid matter). That is because if less than 1% by weight,the effects of dissipation, solidification prevention, and pH adjustmentare insufficient and if exceeding 4% by weight, the effects are scarcelyincreased and on the contrary, the effects are sometimes deteriorated.

[0034] Further, in order to solve the above-described problems, a methodfor producing a cerium-based abrasive has been investigated. Thecerium-based abrasive is produced through the respective steps ofpulverizing raw materials of the cerium-based abrasive, carrying outtreatment with fluorine if necessary, roasting, pulverizing andclassifying the successively treated raw materials. Incidentally, awell-known treatment method to be carried out using hydrofluoric acid,ammonium fluoride and the like can optionally be employed for thetreatment with fluorine.

[0035] Inventors of the invention have made investigations while payingattention to the roasting step among these steps. The roasting step is astep of promoting the particle growth while carrying out sintering ofthe particles and also a step of dissipating fluorine at the same time.Consequently, if both of the particle growth and the fluorine contentcan be controlled in the roasting step, it is desirable for theproduction of the cerium-based abrasive excellent in polishingproperties. Therefore, a roasting step for controlling the dissipationof fluorine has been investigated.

[0036] As a result, it has been found that an abrasive can stably besintered and at the same time an amount of fluorine needed for thechemical polishing can reliably be maintained by controlling thefluorine content after roasting based on the fluorine content beforeroasting. That is, inventors of the invention have found it is possibleto produce the cerium-based abrasive excellent in polishing propertiesand consequently capable of providing excellent polishing evaluations bymeasuring the content of fluorine before and after roasting andcontrolling the decrease amount.

[0037] The invention provides a cerium-based abrasive production methodcomprising a step of roasting raw materials of the cerium-basedabrasive, characterized in that in the case the content of fluorine inrelation to TREO before roasting is defined as F1 (hereinafter simplyreferred as to F1) and the content of fluorine in relation to TREO afterroasting is defined as F2 (hereinafter simply referred as to F2), themethod keeps the ratio F2/F1 in a range 0.7 or higher but 1 or lower.

[0038] Because if F2/F1 is lower than 0.7, the particle growth proceedsexcessively to result in a problem that the amount of coarse particlesto be a cause of formation of polishing scratches is increased. Such aproblem is caused by, for example, a high roasting temperature or a toolong roasting duration. Further, if F2/F1 is lower than 0.7, the costfor the waste gas treatment is problematically increased. Incidentally,unless a fluorine component is supplied, F2/F1 does not exceed 1 and itis generally lower than 1. Further, F2/F1 is preferably 0.75 or higherbut 0.95 or lower. If within the range, roasting can further stably becarried out.

[0039] In this production method, roasting is carried out whileadjusting the roasting temperature, the roasting duration, the gascirculation state in a roasting furnace, and the supply speed to theroasting so as to keep F2/F1 in a range 0.7 or higher but 1 or lower.Hence, the content of fluorine is easily controlled and the polishingproperties and the polishing evaluation can properly be adjusted.Consequently, a cerium-based abrasive can be produced with excellentpolishing properties such as scarcity of scratch formation at the timeof polishing, capability of providing smoothness of the polished face ofan object obtained after polishing, and the like. In such a manner, theproduction method of the invention has a characteristic that anattention is paid only to the fluorine content and its ratio alone iscontrolled but the roasting temperature, the roasting duration, theroasting quantity per time and other variety of operation conditions arenot required to be separately controlled. Such a method is extremelysimple and easy to control the roasting step and preferable.

[0040] Further, investigation has been made regarding the control of thecolor of a cerium-based abrasive by the production method of thecerium-based abrasive. The color of the cerium-based abrasive isaffected by the content of fluorine, the oxidation degree, the specificsurface area expressing the particle size and the like. Consequently,controlling the color of the raw materials of the cerium-based abrasiveto be a desired one by controlling those factors makes it possible toproduce the cerium-based abrasive with excellent polishing properties.However, conventionally, an easy method for controlling the color,especially, a method for easily controlling the color in the roastingstep, has not been made available. For example, as described above,although the color control of the cerium-based abrasive is made possibleby adjusting the roasting temperature, the roasting duration, the gascirculation state in a roasting furnace, and the supply speed to theroasting furnace, it is not necessarily always precise to obtain adesired color of the cerium-based abrasive.

[0041] As a result, the production method of the cerium-based abrasiveof the invention has been found suitable for controlling the color ofthe cerium-based abrasive. This production method, as described above,employs an extremely simple and easy means as the roasting step and itis therefore supposed to possible to relatively easily adjust the colorof the cerium-based abrasive.

[0042] For example, if F2/F1 is controlled to be in a range 0.7 orhigher but 1 or lower by the roasting step in the production method ofthe cerium-based abrasive according to the invention, the cerium-basedabrasive can be produced with a color having an L* value 65 or higherbut 90 or lower in the case of defining the color with the L*a*b* colorsystem. The cerium-based abrasive produced in such a manner, asdescribed above, has advantages that it has a sufficiently highpolishing speed, scarcely causes polishing scratches, and is capable ofproviding a polished surface with excellent smoothness and a highpolishing precision accompanied with little remaining abrasive.

[0043] As raw materials for the cerium-based abrasive to be employed forthe production method of the cerium-based abrasive of the invention,those preferable have the content of praseodymium oxide 1% by weight orhigher and 10% by weight or lower in TREO and the fluorine content F1 inrelation to TREO before roasting 0.5% by weight or higher but 14.3% byweight or lower.

[0044] The fluorine content in relation to TREO of the cerium-basedabrasive can be assumed to be F2 (the fluorine content in relation toTREO after roasting). Consequently, F2 is to be preferable, for example,0.5% by weight or higher but 10% by weight or lower. In this case,taking the production condition, “F2/F1 is 0.7 or higher and 1 orlower”, as described above into consideration, it is preferable for F1to be 0.5% by weight or higher but 14.3% by weight or lower. Further, inorder to keep F2 more reliably be 0.5% by weight or higher but 10% byweight or lower, F1 is preferable to be 0.7% by weight or higher but 10%by weight or lower. The reasons for the preference for such ranges areas described above. Further, in the production of the cerium-basedabrasive, in th case of a high fluorine content, waste gas treatment atthe time of roasting is required and a problem of corrosion of furnacematerials inconveniently takes place. In consideration of such a pointof view, F1 is preferable to be 10% by weight or lower.

[0045] Controlling F2/F1 in the roasting step and at the same timecontrolling the content of praseodymium oxide in TREO of raw materialsand the fluorine content F1 in relation to TREO before roasting make iteasy to produce the abrasive having not only an L* value in a range 65or higher and 90 or lower but also an a* value 0 or higher and 15 orlower and a b* value 10 or higher and 30 or lower.

[0046] Further, in the case the fluorine content in relation to TREO inthe raw materials for the cerium-based abrasive is lower than 0.5% byweight, or even if it is 0.5% by weight or higher, in the case thefluorine content in relation to TREO is wanted to be increased, thetreatment with fluorine as described above is carried out to control F10.5% by weight or higher but 14.3% by weight or lower. Incidentally, thecontent of TREO in the cerium-based abrasive to be produced, thecontents of cerium oxide and praseodymium oxide in TREO, and further thefluorine content are as described above.

[0047] The contents of cerium oxide and rare earth oxides such aspraseodymium oxide in TREO can be assumed to be unchanged between thosein the raw materials for the cerium-based abrasive and the cerium-basedabrasive. Consequently, those having the contents of cerium oxide andpraseodymium oxide in TREO within the ranges of the values defined to bepreferable in the cerium-based abrasive are preferable to be used as rawmaterials for the cerium-based abrasive. Incidentally, the content ofTREO in the raw materials sometimes becomes lower than 80% by weightdepending on the contents of water and carbonate residue in the rawmaterials, however usually since impurities are slight, the content ofTREO reaches 80% by weight or higher after roasting and therefore itdoes not cause a problem.

[0048] Further, in the roasting step, it is preferable that the roastingtemperature is 600° C. or higher but 1,200° C. or lower and the roastingduration is 1 hour or longer but 60 hours or shorter.

[0049] In the roasting, cerium-containing particulate rare earthelements as raw materials are subjected to particle growth to a sizeenabling proper polishing speed. As the roasting means, for example, anelectric furnace, a rotary kiln, and the like can be employed. Theambient atmosphere is preferable to be oxidative atmosphere and, forexample, the atmospheric air can be used as the ambient atmosphere.Generally, if the roasting temperature is high, abnormal particle growtheasily takes place and the amount of coarse particles in the abrasive iseasy to be increased, however it has been found as a result of theinvestigation that praseodymium compounds including praseodymium oxidesuppress the abnormal particle growth and at the same time promotes evenroasting of the raw materials to be effective to suppress the amount ofthe coarse particles. Consequently, if raw materials containingpraseodymium compounds including praseodymium oxide are used, roastingcan be carried out at a higher temperature. If roasting is carried outat a high temperature, sintering is easy to be promoted, so that it isespecially effective in the case an abrasive with a large averageparticle size is obtained and in the case an abrasive with a largeaverage particle size and a sufficiently high polishing speed. Yet thecontent of praseodymium compounds including praseodymium oxide in theraw materials is not particularly restricted, the proper content is togive the content of praseodymium oxide within a range 1% by weight orhigher but 10% by weight or lower in TREO in the abrasive. Incidentally,the color of the cerium-based abrasive is affected by the roastingtemperature, the above described temperature range is preferable fromthis point of view.

MODES FOR CARRYING OUT THE INVENTION

[0050] Hereinafter, practical embodiments of the invention will bedescribed.

[0051] At first, raw material slurries were obtained by pulverizing rawmaterials for cerium-based abrasives by a wet type ball mill.Incidentally, as the raw materials, bastnasite concentrate, rare earthoxides, and cerium oxide were used. After that, no fluorine treatmentwas carried out for Example 1, Example 2, and Comparative Example 2. Onthe other hand, fluorine treatment was carried out respectively in orderto increase the fluorine content for Example 3 and Comparative Example1, in order to add fluorine to the rare earth oxides as raw materialsfor Examples 4 to 6 and Comparative Example 3, and in order to addfluorine to cerium oxide for Comparative Example 4. After that, theslurries were filtered and dried at 120° C to obtain powders. Therespective powders obtained after drying were roasting in a range of 800to 1,100° C. for 3 hours. Then, the powders obtained after roasting werepulverized and classified in order to decrease coarse particles with asize of 10 μm or larger as much as possible and thus cerium-basedabrasives were obtained.

[0052] The contents of cerium oxide (CeO₂) and praseodymium oxide(Pr₆O₁₁) before roasting, the fluoride content (<F1>) before roasting,the fluoride content (<F2>) after roasting, the content of TREO afterroasting, and the yield (<F2/F1>) of fluoride by roasting were measured.The measurement results were shown in Table 1.

[0053] The fluorine content is a value measured by an alkalinemelting-warm water extraction-fluoride ion electrode method. That is ameasurement method as follows. At first, an abrasive or raw materials ofthe abrasive as specimens were melted with an alkaline melting agent,subjected to extraction with warm water after being cooled spontaneouslyand made to be constant in volume. A proper amount of the resultingspecimen was sampled, mixed with a buffer solution and then adjusted tohave pH about 5.3 and made to be constant in volume to obtain a samplesolution. Also, standard solutions were prepared. The operation forobtaining the standard solutions was same as that for obtaining thesample solution except that no specimen was used and a fluoride ionstandard solution was added after the sampling to adjust the fluorideion content to be a desired value. Several kinds of standard solutionswere obtained by changing the fluoride ion concentration. Regarding thestandard solutions and the sample solutions obtained in such a manner,the respective fluoride ion contents were measured using an ion meterequipped with a fluoride ion electrode. More practically, the fluorideion concentrations of the sample solutions were obtained based on acalibration curve obtained by measurement for the standard solutions andthe obtained fluoride ion concentrations were converted into thefluorine contents of specimens. Further, the obtained fluorine contentswere divided with the TREO contents of specimens described below toobtain the fluorine content in relation to TREO. Incidentally, thefluorine contents before roasting were measured by drying specimensbefore roasting at 120 ° C. for 2 hours and subjecting the resultingspecimens to the same operation as that carried out for the abrasivesafter roasting.

[0054] The TREO contents were measured as follows. At first, specimenswere decomposed using perhydrochloric acid and hydrogen peroxide andthen oxalic acid was added to the resulting solutions and pH of thesolutions was kept about 1.5 to obtain precipitates. The precipitateswere filtered and obtained precipitates were roasted at 1,000° C. Theweights of the roasted materials were measured and the TREO content wascalculated based on the weight of the roasted materials in relation tothe weights of the specimens.

[0055] Cerium oxide and praseodymium oxide contents were measured usingan alkaline melting agent and ICP emission spectrophotometry. At first,specimens were dissolved in an acid or alkaline melted and then therespective specimens were sampled in a proper amount and made to beconstant in volume to obtain samples. Also, several kinds of standardsamples with changed concentrations of cerium or the like were prepared.The contents of cerium or the like of the samples were quantitativelymeasured based on the calibration curve showing the concentrations ofcerium or the like obtained by measurement for the standard samples, andwere converted into the cerium oxide and praseodymium oxide contents.The cerium oxide and praseodymium oxide contents in TREO were calculatedfrom the cerium oxide and praseodymium oxide contents in the respectivesamples and the contents of TREO in the samples. TABLE 1 CHEMICALANALYSIS RESULTS AND YIELD OF FLUORINE Chemical analysis before RoastingChemical analysis after roasting step roasting Pr₆O₁₁/ F/TREO RoastingF/TREO Yield of CeO₂/TREO TREO <F1> temperature TREO <F2> fluorine Rawmaterial (wt %) (wt %) (wt %) (° C.) (wt %) (wt %) <F2/F1> Example 1bastnasite 50 3 8 800 83 6.7 0.84 Example 2 bastnasite 50 3 8 900 85 6.00.75 Example 3 bastnasite 50 3 10 1000 89 7.1 0.71 Example 4 rare earth60 5 8 850 90 7.1 0.89 oxides Example 5 rare earth 60 5 8 900 92 6.80.85 oxides Example 6 rare earth 60 5 8 1000 93 6.1 0.76 oxidesComparative bastnasite 50 3 15 1000 84 7.3 0.49 Example 1 Comparativerare earth 60 5 <0.1 900 97 <0.1 —¹⁾ Example 2 oxides Comparative rareearth 60 5 8 1100 94 4.3 0.54 Example 3 oxides Comparative cerium oxide99 0 3 800 99 <0.1 <0.03   Example 4

[0056] As it is made clear from Table 1, in Examples 1 to 6, the yieldof fluorine (F2/F1) was 0.7 or higher. It was supposedly owing to theeffect attributed that the contents of cerium oxide, praseodymium oxideand fluorine were in respectively prescribed ranges. On the other hand,in Comparative Example 1, the yield was low. It was supposedly owing toexcess fluorine relative to praseodymium oxide. In Comparative Example3, the yield of fluorine was low. It was supposedly attributed to thehigh roasting temperature. In Comparative Example 4, almost all fluorinewas dispersed and dissipation of fluorine could not be prevented byroasting. It was supposedly attributed to no content of praseodymiumoxide.

[0057] Further, the cerium-based abrasives obtained in the respectiveexamples and Comparative Examples were subjected to color measurement(by a color and color difference meter: CR-300, manufactured by MinoltaCo., Ltd.) and the specific surface area measurement (by a BET specificsurface area measurement apparatus using nitrogen gas: Multisorp,manufactured by Uasa Ionics Co.). Further, a polishing test was carriedout to measure the polishing properties of finally obtained cerium-basedabrasives. The measurement results and the results of the polishing testwere shown in Table 2. Incidentally, although the color measurement wascarried out after pulverization after the roasting, in other words,before the classifying step, the color of each abrasive product wasfound almost the same before and after the classifying step, so that thecolors of the cerium-based abrasives wer shown in Table 2.

[0058] Incidentally, in the polishing test, an Osker type polishingmachine (HSP-21 model, manufactured by Taito Seiki Co., Ltd.) wasemployed. For the polishing test, cerium-based abrasive slurries havingthe abrasive concentration of 10% by weight and prepared by dispersingthe powder type cerium-based abrasives in water were used. The object tobe polished was a glass material for a flat panel with 65 mmφ and apolishing pad made of a polyurethane was used. The polishing conditionswere as follows: the rotation speed of the glass material was 1,700 rpm;the pushing pressure of the pad was 98 kPa (1 kg/cm²), and the polishingduration was 10 minutes.

[0059] The polishing values in Table 2 were the values based on thepolishing amounts calculated by measuring the weight of the glassmaterial before and after polishing. They were expressed as relativevalues in the case the value of Example 1 was set to be 100.

[0060] The polishing scratches were evaluated as follows. At first lightof a halogen lamp with 300,000 lux light source was radiated to thesurface of the glass for a flat panel after polishing and the number andthe size of the scratches were judged by a reflection method. Then,corresponding to the number and the size of the scratches, the resultswere numerary expressed by a minus-point system on the basis of 100marks. In Table 2, “E” showed to be 95 or higher and 100 or lower marksand extremely suitable for precision polishing; “G” showed to be 90 orhigher but lower than 95 marks and suitable for precision polishing; “F”showed to be or higher 80 and lower than 90 marks and usable for generalpolishing; and “P” showed to be lower than 80 and unsuitable as anabrasive, respectively.

[0061] The evaluation of the remaining abrasives was carried out asfollows. At first, the glass after polishing was washed in pure waterusing an ultrasonic washing apparatus and dried in dust-free state.After that, the glass surface was observed by an optical microscope andthe existence of the residual abrasives adhering to the glass surfacewas observed. In Table 2, “E” showed the residual abrasives werescarcely observed to prove extreme suitability as an abrasive; “G”showed the residual abrasives were slightly observed to provesuitability as an abrasive; and “P” showed many residual abrasives wereobserved to prove unsuitability as an abrasive, respectively. TABLE 2COLOR, THE SPECIFIC SURFACE AREA, AND POLISHING PROPERTIES OFCERIUM-BASED ABRASIVE Physical measurement after roasting Evaluation ofpolishing Measured color of powder Specific surface Polishing PolishingResidual L* value a* value b* value area (m²/g) value scratch¹⁾abrasive²⁾ Example 1 85.52 4.86 17.64 7.9 100 E G Example 2 77.87 5.9420.78 2.5 180 E E Example 3 74.53 8.76 22.45 1.6 330 G E Example 4 75.4811.27 19.84 6.1 120 E G Example 5 74.12 12.55 21.29 3.5 155 E E Example6 72.23 14.90 24.12 2.9 170 E E Comparative Example 1 64.43 16.85 18.871.0 400 P E Comparative Example 2 49.74 11.05 9.36 13.2 25 P PComparative Example 3 60.19 19.41 19.28 0.7 450 P E Comparative Example4 94.53 −1.95 2.40 9.5 75 G P

[0062] As shown in Table 2, in Examples 1 to 6, the L* values in theL*a*b* color system were all in a range 65 or higher but 90 or lower:the a* values were all in a range 0 or higher but 15 or lower: b* valuesall in a range 10 or higher but 30 or lower: and the specific surfacearea was 1 to 30 m²/g, proving any of the polishing values, thepolishing scratches, and the residual abrasives, which were polishingproperties, to be excellent.

[0063] However, in Comparative Example 1, fluorine was found existingexcessively and the specific surface area was small and the polishingscratches were many. The b* value was 18.87, whereas the L* value waslower than 65 and the a* value was higher than 15.

[0064] In Comparative Example 2, the specific surface area was high andthe polishing value was extremely low and the residual abrasive was highin quantity. It was supposedly attributed to scarce existence offluorine. The L* value was 49.74, lower than 65 and the a* value was1.05, lower than 15, and further the b* value was 9.36, lower than 10.

[0065] In Comparative Example 3, many polishing scratches were observed.The b* value was 19.28, whereas the L* value was 60.19, lower than 65and the a* value was 19.41, higher than 15.

[0066] In Comparative Example 4, the residual abrasive was high inquantity. The L* value was 94.53, higher than 90. It was supposedlyattributed to no existence of praseodymium oxide. Further, the a* valuewas −1.95 with the inverse sign showing the color change toward greenand also the b* value was 2.40, extremely small as compared with 10.

INDUSTRIAL APPLICABILITY

[0067] The invention can provide a cerium-based abrasive excellent inthe polishing properties; a sufficiently high polishing speed and scarceprobability of causing polishing scratches and leaving little remainingabrasive. Further, the invention provides a production method of such acerium-based abrasive. The abrasive provided can be used for surfacepolishing with a high precision required in production of glasssubstrates for optical disks and magnetic disks and the like.

1. A cerium-based abrasive containing cerium oxide as a main component,wherein an L* value is 65 or higher but 90 or lower when a color of thecerium-based abrasive is expressed by an L*a*b* color system.
 2. Thecerium-based abrasive according to claim 1, wherein the a* value in theL*a*b* color system is 0 or higher but 15 or lower.
 3. The cerium-basedabrasive according to claim 1 or claim 2, wherein the b* value in theL*a*b* color system is 10 or higher but 30 or lower.
 4. The cerium-basedabrasive according to any one of claim 1 to claim 3, wherein a contentof total rare earth oxides (TREO) in the cerium-based abrasive is 80% byweight or higher but 99% by weight or lower and a content of fluorinerelative to TREO is 0.5% by weight or higher but 10% by weight or lower.5. The cerium-based abrasive according to claim 4, wherein the contentof praseodymium oxide in TREO is 1% by weight or higher but 10% byweight or lower.
 6. The cerium-based abrasive according to any one ofclaim 1 to claim 5, wherein a specific surface area of the cerium-basedabrasive particles is 1 m²/g or larger but 30 m²/g or smaller.
 7. Aproduction method of a cerium-based abrasive containing cerium oxide asa main component comprising a step of roasting raw materials for thecerium-based abrasive, wherein a ratio F2/F1 is 0.7 or higher but 1 orlower when a content of fluorine in relation to TREO before roasting isdefined as F1 and a content of fluorine in relation to TREO afterroasting is defined as F2.
 8. The production method of a cerium-basedabrasive according to claim 7, wherein raw materials for thecerium-based abrasive have a content of praseodymium oxide in TREO 1% byweight or higher but 10% by weight or lower and a content of fluorine toTREO before roasting F1 0.5% by weight or higher but 14.3% by weight orlower.
 9. The production method of a cerium-based abrasive according toclaim 8, wherein roasting temperature of the roasting step is 600° C. orhigher but 1,200° C. or lower and roasting duration is 1 hour or longerbut 60 hours or shorter.